Iron-based alloy composition, parts produced from this composition and production method

ABSTRACT

An iron-based alloy composition includes 0.28-0.34% C, max 0.25% Si, max 0.8% Mn, 0.85-0.95% Cr, 1.10-1.50% Ni, 0.41-0.50% Mo, 0.001-0.007% B, 0.002-0.03% Nb, and balanced amount of Fe and inevitable impurities. Moreover, parts with a hardness of at least 480 HB, a tensile strength of at least 1700 MPa, a total elongation of at least 7% and an impact strength of at least 16 J are obtained by the iron-based alloy composition and a production method of the parts.

CROSS REFERENCE TO THE RELATED APPLICATIONS

This application is a continuation application of InternationalApplication No. PCT/TR2021/050790, filed on Aug. 11, 2021, which isbased upon and claims priority to Turkish Patent Application No.2020/18497, filed on Nov. 18, 2020, the entire contents of which areincorporated herein by reference.

TECHNICAL FIELD

The invention relates to an iron-based alloy for hot forming process,the parts obtained from this composition and production method.

The invention relates to an armor steel chemical composition andproduction method to be used for production of armored vehicles, armoredparts and armored buildings in order to be protected against variousammunitions. This production method particularly relates to hot formingprocess and heat treatment of armor parts in three-dimensional geometry.

BACKGROUND

Armor steels are produced via heat treatment after hot rolling process.Ballistic protection feature is achieved after this heat treatmentprocess. Armor steels are defined in MIL-DTL-46100E standard, one of thestandards specifying armor steels, and their chemical compositionpercentage by weight are 0.32% maximum carbon, 0.03% maximum boron,0.010% maximum sulfur, and 0.02% maximum phosphorus; and it is statedthat the manganese, nickel, chromium, and molybdenum elements are notcompulsory. Specifically designated armor steels such as Armox 500 andSecure 500 have been produced for years. These alloys include Mn, Cr,Ni, and Mo. Chemical composition and production methods have beenstudied in different studies metallurgically.

For example; in the patent numbered EP1052296B1, the use of a steelcontaining 0.15-0.20% carbon, 0.10-0.20% silicon, 0.70-1.70% manganese,<0.02% phosphorus, <0.005% sulfur <0.01% nitrogen, 0.009-0.10% aluminum,0.50-1.00% chromium, 0.20-0.70% molybdenum 1.00-2.50% nickel, 0.05-0.25%vanadium, 0.0050% boron, and iron and inevitable impurities for themanufacture of an armor plate is described.

For example; an armor steel with ballistic protection feature wasdeveloped in the patent numbered RU2236482C1 by providing a differentelement distribution (0.46-0.54% carbon, 0.17-0.37% silicon, max 0.5%manganese, 2.80-3.20% chromium, 1.50-2.00% nickel, 1.70-2.20%molybdenum, 0.25-0.35% vanadium, 0.01-0.03% aluminum, max 0.012% sulfur,max 0.012% phosphorus). This steel was produced by hot forging, surfacecourse removing, and hot rolling. Heat treatment was applied after hotrolling.

Armor steel with a different chemical composition (0.29-0.38% carbon,0.15-0.37% silicon, 0.30-0.60% manganese, 1.20-2.00% chromium,1.20-2.20% nickel, 0.72-0.90% molybdenum, 0.06-0.20% vanadium,0.01-0.05% aluminum, 0.005-0.020% nitrogen, max 0.50% copper, max 0.05%niobium, max 0.012% sulfur, max 0.015% phosphorus and iron in the rest)was developed in the patent numbered RU2341583C2.

Armor steel with a different chemical composition (0.28-0.40% carbon,0.80-1.40% silicon, 0.50-0.80% manganese, 0.10-0.70% chromium,1.50-2.20% nickel, 0.30-0.80% molybdenum, 0.005-0.05% aluminum, max0.30% copper, max 0.012% sulfur, max 0.015% phosphorus, and iron and0.8-2.0% molybdenum/carbon in the rest) was developed in the patentnumbered RU2520247C1.

Armor steel with a different chemical composition (0.12% <0.20 carbon,0.8-2.5% manganese, 0.01-0.05% aluminum, ≤1.0% silicon, and preferably<1.0% chromium, <0.009% nickel, 0.015-0.18% titanium, 0.0020-0.0040boron, and iron and inevitable impurities in the rest) was developed inthe patent numbered DE10220476B9. The hardness of this steel is below400 HB. Its tensile strength is over 800 MPa.

A different chemical composition (0.20-0.40% carbon, 0.05-0.50% silicon,0.50-1.50% manganese, max 0.015% phosphorus, 0.003-0.10% niobium,0.0003-0.010% boron, 0.003-0.30% aluminum, 0.0005-0.010% nickel,0.05-1.50% copper, 0.05-2.00%, nickel, 0.10-2.00% chromium, 0.05-1.50%molybdenum, 0.003-0.20% vanadium, 0.003-0.10% titanium, and Fe andinevitable impurities in the rest) was developed in the patent numberedJPH09118950A. The steel is produced after the plate with relevantcomposition is heated to 1250° C. or to a lower temperature, hot rolledand cooled, and is reheated to a temperature above Ac3, and cooled at arate of 1.5° C./s subsequently.

High hardened steel with a new chemical composition (0.25-0.45% carbon,0.01-1.5%, silicon, 0.35%-3.0% manganese, 0.5-4.0% nickel, 0.01-1.2%aluminum, max 2.0% chromium, max 1.0% molybdenum, max 1.5% copper, max0.5% vanadium, max 0.2% niobium, max 0.2% titanium, max 0.01% boron, max0.01% calcium) was developed in the patent numbered EP2789699A1. Thehardness of this steel is above 450 HB and the previous austenite grainsare oriented in the rolling direction to have a minimum aspect ratio of1.2.

High-hard steels exemplified above can be used as armor steel. Althoughhaving different compositions, these steels are generally produced bymassive forming methods such as plate forging and hot rolling. Ballisticfeatures such as high hardness are acquired via subsequentaustenitization, rapid cooling, and tempering heat treatment.

Patent numbered U.S. Pat. No. 9,121,088B2, registered by ATI PropertiesLLC, has completely changed the chemical composition of typical armorsteel and provides the steel with ballistic protection via air coolingwithout the need for quenching and tempering processes afteraustenitization. Patent numbered U.S. Pat. No. 9,657,363B2 providesballistic protection by tempering after air cooling. Chemicalcompositions used in armor steels produced with air cooling are asfollows:

U.S. Pat. No. 9,121,088B2 (0.48-0.52% carbon; 0.15-1.00% manganese;0.15-0.45% silicon; 0.95-1.70% chromium; 3.30-4.30% nickel; 0.35-0.65%molybdenum; 0.0008-0.0030% boron; 0.001-0.015% cerium; 0.001-0.015%lanthanum; max 0.002% sulfur; max 0.015% phosphorus; 0.10% nitrogen;iron and inevitable impurities in the rest)

U.S. Pat. No. 9,657,363B2 (0.18-0.26% carbon; 3.50-4.00% nickel;1.60-2.00% chromium; max 0.50% molybdenum; 0.80-1.20% manganese;0.25-0.45% silicon; 0.005% titanium; 0.020% phosphorus; max % 0.005boron; max 0.003% sulfur; iron; and inevitable impurities in the rest).

As is seen, both of these patents include a high amount of Nickel.Nickel amplifies the hardenability of the steels and facilitates theconversion into martensite. The use of nickel is limited due to its highcost.

All of the above-mentioned steels are produced after a process of hotrolling or plate forging. Forming the steels mentioned in the currenttechnique into a three-dimensional geometry implies great challenges dueto their high hardness. In general, parts are cut from these armor steelplates via water jet, laser, plasma, etc. methods and weld bondedafterward. The ballistic protection feature is lost due to heat input inthe weld zones after weld bonding. For this reason, additional armorsteels are adjoined to the back of these zones. This causes an increasein the weight of the parts. Some parts are cold formed to a certainextent. Cold forming, which increases energy and initial investmentcosts, is generally avoided as high press forces are required.

In the patent numbered U.S. Pat. No. 9,671,199B1, an innovative methodwas developed by Premier Body Armor LLC so as to avoid theabove-mentioned problems. In this method, armor steel plates are cut indesired geometries, annealed at austenitizing temperature, placed inpreviously produced forming tools, and formed between the tools byapplying press force. The next step involves re-annealing of thethree-dimensional product at austenitizing temperature and rapidcooling. Tempering is the last step of the process. Cooling is appliedduring tool forming in a way that is slower than air cooling. Thementioned method is a long process and increases energy costs, whileresolving the aforementioned forming problems. Heating the steel forforming purposes and reheating it to gain ballistic feature can causemany problems. The first of these problems is the decarburization.Double austenitization may cause excessive decarburization on thesurface. Double heating and cooling, on the other hand, increasesthermal stress. Additionally, heat treatment of the product on theoutside of the tool after forming process may cause distortions in thestructure and cause deformation. One of the most crucial issues is thatdouble heat treatment increases the production cost.

Hot formed steel is mentioned in the patent numbered EP2341156B1. Itschemical composition includes 0.15-0.35% carbon, 0.8-2.5% manganese,1.5-2.5% silicon, max 0.4% chromium, max 0.1% aluminum, max 0.3% nickel,0.0008-0.1% boron, 0.005-0.1% titanium, max 0.1% niobium, and iron andinevitable impurities in the rest. The amount of manganese and siliconis high, while the amount of nickel and chromium is lower than thedesired amounts to provide steel with the ballistic feature.

A specific steel type is developed for the production of tube-formedsteels in the patent numbered EP1961832B1, and its chemical compositionincludes high amount of silicon and carbon. Its composition isinsufficient in terms of armor steel production. Its chemicalcomposition includes 0.40-0.44% carbon, 1.5-2.2% silicon, 0.3-0.8%manganese, 1.1-1.5% chromium, 0.004-0.015% nitrogen, 0.02-0.04% niobium,0.01-0.015% vanadium, 0.002-0.004% boron, and iron in the rest, andincludes 0.015% phosphorus, max 0.01% sulfur, max 0.2% nickel, max 0.1%copper, max 0.02% tin, max 0.015% aluminum, max 0.01% titanium, max0.08% molybdenum as conventional impurities.

A steel type that can be coated with nitride before forming wasdeveloped in the patent numbered U.S. Pat. No. 9,200,358B2 in order toprevent decarburization during hot forming. Its chemical compositionincludes 0.22-0.25% carbon; 0.10-0.50% silicon; 1.00-2.50% manganese;max 0.025% phosphorus; max 0.010% sulfur; 0.010-0.060% aluminum;0.0015-0.005% boron; 0.10-0.80% chromium; 0.020-0.050% titanium; max0.50% molybdenum; max 0.10% copper; max 0.30% nickel; and iron andpost-production elements in the rest. The manganese amount of this steelis high, while the carbon amount is low. Manganese segregation occurs inthe production of steels with high manganese content and makes it hardto have a homogenous structure in this regard. For this reason, it ishard to produce armor steel in high manganese content. The carbon amountof this steel is not sufficient to provide required hardness for thearmor property. The amount of nickel and chromium is lower than theamounts required for ballistic protection features.

Steel produced by hot forming is mentioned in the patent numberedES2336967T3. Its chemical composition includes 0.18-0.30% carbon,0.1-0.7% silicon, 1.0-2.5% manganese, 0.025% phosphorus, max 0.01%sulfur, 0.1-0.8% chromium, 0.1-0.5% molybdenum, 0.02-0.05% titanium, %0.002-0.005 boron, 0.01-0.06% aluminum. Composition, however, does notinclude nickel required to provide the expected high hardness andtoughness relevance. Its manganese ratio is high.

Hot formed steel is mentioned in the patent numbered DE102005014298B4.Its chemical composition includes 0.2-0.4% carbon, 0.3-0.8% silicon,1.0-2.5% manganese, max 0.020% phosphorus, max 0.05% sulfur, 0.1-0.5%chromium, 0.1-1.0% molybdenum, max 2% copper, % 0.1-1.0 nickel,0.001-0.01% molybdenum, 0.001-0.01% boron, max 0.05% aluminum, 0.01-1%tungsten, max 0.005% nitrogen. The amount of silicon is high, while theamount of chromium is low. Therefore, similar to the patent mentionedabove, said chemical composition is not suitable for the production ofdesired armor steel.

The production of hardened steel parts by hot forming is mentioned inthe patent numbered DE102008010168B4. Its chemical composition includes0.35-0.55% carbon, 0.1-2.5% silicon, 0.3-2.5% manganese, max 0.05%phosphorus, max 0.01% sulfur, max 0.08% aluminum, max 0.5% copper,0.1-2.0% chromium, max 3.0% nickel, max 1.0% molybdenum, max 2.0%cobalt, 0.001-0.005% boron, 0.01-0.08% niobium, max 0.4% vanadium, max0.02% nitrogen, max 0.2% titanium. This composition is distinctivelyhigh in carbon. High carbon amount decreases weldability. It alsoincreases distortion formation due to heat treatment during coolingprocess.

The production of hardened steel parts by hot forming is mentioned inthe patent numbered DE102012109693B4. Its chemical composition includes0.29-0.32% carbon, 0.35-0.45% silicon, 0.8-0.9% manganese, max 0.015%phosphorus, max 0.003% sulfur, 0.01-0.03% aluminum, 0.8-0.95% chromium,0.3-0.4% molybdenum, % 1.0-1.65 nickel, max 0.15% copper, max 0.1%titanium, 0.002-0.003% boron, 0.02-0.03% niobium, max 0.012% nitrogen,0.002-0.55% cobalt. Weldability of the steel is sufficient owing to itscarbon amount. Its silicon amount, however, is high. Silicon oxideformation occurs on the surface after heat treatment is applied to thesteels with high carbon content at elevated temperatures. These oxides,known as red scale in the industry, cannot be removed after heattreatment. Therefore, it both reduces the commercial value of the steeland decreases the paint coating workability by causing surface defects.Furthermore, these oxides damage the tool itself during the coolingprocess. Problems related to high amount of silicon in armor steelproduction under normal conditions can therefore be eliminated while itis difficult to avoid the negative effects of silicon if it is aimed toproduce parts by hot forming. Another issue is that the manganese amountof the alloy is above the desired values in order to preventsegregation. Molybdenum amount is also within the limits of adequatehardenability.

The methods mentioned in the abovementioned patents numberedEP1052296B1, RU2236482C1, RU2341583C2, RU2520247C1, DE10220476B9,JPH09118950A, EP2789699A1 are related to developing armor steel. Armorsteels produced with relevant methods, however, are produced withconventional production methods. These production methods are hotforging, hot rolling and heat treatment after casting to gain armorfeature to the steel.

Methods elaborated in the patents numbered U.S. Pat. No. 9,121,088B2 andU.S. Pat. No. 9,657,363B2 similarly eliminate the heat treatment stageafter production. Ballistic protection feature is achieved during aircooling process. Notwithstanding, this process is costly because itincludes high-alloy elements.

Armor steels produced with the abovementioned methods are produced asplates with post-heat treatment ballistic protection feature. Armoredvehicles are cut and welded to the desired dimensions by themanufacturers, and 3-dimensional parts are produced thereafter. This isbecause their forming is limited due to their high hardness. Armorfeature is lost during welding process due to temperature action.Additional armored parts are adjoined behind the weld zones herewith.This increases the weight. Furthermore, it also prevents armored vehicleor armored equipment designers from freely devising against possiblethreats. Hereby, even if a design is made in complex geometry,production of the relevant design will be impossible. Regardless,geometrical properties of the armor materials have great importance forprotection against threats. For example, parts with differentgeometrical properties against the incoming threat show differentballistic resistance depending on the counterbalancing angle.

Production of armor steel parts via hot forming might offer a propersolution for the abovementioned problems. Designers will be able todesign parts with different geometries. Thus, armored vehicles withimproved aerodynamic structure will be able to be produced. In thiscase, three-dimensional parts of the desired geometry can be produceddirectly, instead of welding many different armor steels; thereby,vehicle weights can be reduced, and maneuverability can be increased.

Nevertheless, the production of armor steel by hot forming is not aneasy process as it appears to be. Patents numbered EP2341156B1,EP1961832B1, U.S. Pat. No. 9,200,358B2, ES2336967T3, DE102005014298B4relates to part production with hot forming. Alloys developed for hotforming are insufficient in terms of ballistic resistance after formingprocess. The reason is that alloying elements must be selectedmeticulously in order to gain armor feature to steel. Armor featurerequires a complete optimization of the material's hardness, yieldingstrength, tensile strength, impact toughness. It is also preferred thatthe armor steels do not contain segregation and anisotropy. Chemicalcompositions developed in the patents numbered EP2341156B1, EP1961832B1,U.S. Pat. No. 9,200,358B2, ES2336967T3, DE102005014298B4 are notsufficient to provide the threat resistance properties expected of anarmor material with effective thickness.

Armor steel production with hot forming is mentioned in the patentnumbered DE102008010168B4. Nevertheless, carbon amount is between0.35-0.55 in this patent. Welding the armor steels with this amount ofcarbon with other parts in the vehicle production causes defects in theweld zone. For this reason, this process is not desired by armoredvehicle manufacturers. In addition to this, cracks may occur in the tooldue to thermal stress during cooling process.

Manganese amount of the developed armor steel in the patent numberedDE102012109693B4 is between 0.8-0.9%. It is known that steels containingmanganese in these intervals transform into an inhomogeneous structuredue to manganese segregation after hot rolling. Even though manganeseincreases hardenability, it creates difficulties during productionprocess. Silicon content is high in the mentioned patent and many otherpatents relate to armor steel (U.S. Pat. No. 9,121,088B2, U.S. Pat. No.9,657,363B2, DE102005014298B4, ES2336967T3, EP2341156B1). This causesoxide formation on the surface after hot rolling or during heattreatment, depending on the silicon content in the steel. This oxidationcannot be removed by acid and sanding. It, therefore, constitutes aproblem for later coating or painting. Fayalite (Fe₂SO₄) is formedduring hot rolling of steels containing silicon, and bonds with FeO.Since this is a strong bond, it makes it difficult to remove the oxideand causes the formation of red scale. The scale cannot be removed bytraditional oxidizing methods, causing defects in these areas anddecreasing the paint coating workability. In a study evaluating thesurfaces of the hot rolled steels with different silicon contents viaheat treatment, it was observed that if the silicon amount is below0.25%, Fe₂SO₄ compound does not form, which causes the formation of redscales (Fukagawa, T., Okada, H., & Maehara, Y. (1994). Mechanism of redscale defect formation in Si-added hot-rolled steel sheets. ISIJinternational, 34(11), 906-911.)

As a result, due to abovementioned drawbacks and the insufficiency ofpresent solutions in the art, it is necessary to make an improvement inthe related technical field.

SUMMARY

The present invention relates to hot-formed armor steel composition andproduction method that provides the above-mentioned requirements,eliminates all disadvantages and implies certain additional advantages.

The object of the invention is to develop an alloy composition that willmake it possible to produce armor steel by hot forming and todemonstrate the production method using this composition. It is aimedthat the developed alloy can be able to martensitic transformation atlimited cooling rates that can be applied during hot forming, be amaterial with paint coating workability by minimizing the oxide layerson its surface, and be easily cut into desired geometries before hotforming. Hence, solid armor steel parts are designed in desiredgeometries, armored vehicles are produced with enhanced aerodynamics,and costs of energy and production are reduced.

The invention involves iron-based alloy composition for obtaininghot-formed armor steel, the use of this composition, hot-formed armorsteel parts obtained with the use of this composition, and theproduction management of these parts for the fulfillment of theobjectives explained above.

Composition subject to invention basically includes 0.28-0.34% carbon,max 0.25% silicon, max 0.8% manganese, 0.85-0.95% chromium, 1.10-1.50%nickel, 0.41-0.50% molybdenum, 0.001-0.007% boron, 0.002-0.03% niobiumand iron and inevitable impurities in balanced amount.

Embodiments of the invention include one or more elements selected fromthe group containing trace amounts of phosphorus, sulfur, copper,aluminum, tungsten, cobalt, titanium, oxygen, hydrogen, nitrogen asinevitable impurities.

Armor steel obtained from the combination of the invention has ahardness of at least 480 HB, tensile strength of at least 1700 MPa,total elongation of at least 7% and/or impact strength of 16 J, andincludes at least 90% martensite in its microstructure.

The invention is the production method of hot formed armor steel with anaim of achieving the abovementioned objectives, and

involves the foregoing process steps: ingot or slab casting of the alloyincluding 0.28-0.34% carbon, max 0.25% silicon, max 0.8% manganese,0.85-0.95% chromium, 1.10-1.50% nickel, 0.41-0.50% molybdenum,0.001-0.007% boron, 0.002-0.03% niobium, and balanced amounts of ironand inevitable impurities,

hot rolling the slab or ingot into a plate,

plate cooling and cutting,

applying primary heat treatment to cut plates,

Forming the heated plates by pressing in the cooled tool,

applying secondary heat treatment to formed steel parts

An embodiment of the invention involves one or more elements selectedfrom the group containing trace amounts of phosphorus, sulfur, copper,aluminum, tungsten, cobalt, titanium, oxygen, hydrogen, nitrogen (i) asimpurities unsolicited alloy in the process steps.

An embodiment of the invention involves (ii) heating the slab or ingotto above 1050° C. for at least 4 hours in the foregoing process step.

An embodiment of the invention involves (iii) the cooling of the platesdown to 2° C./s or slower, microstructure of ferrite+perlite, bainite,or a mixture of these phases and obtaining a plate with a hardness scalebelow 300 HB, a heating process of the plate above 300° C. andtransforming its microstructure into tempered martensite, provided thatthe process is performed faster without cooling.

An embodiment of the invention involves (iv) primary heat treatment ofplates cut in the foregoing process step by heating them to atemperature below 1000° C. and above AC3 for at least 10 minutes.

An embodiment of the invention involves (v) forming the plates bycooling them to a temperature of 300° C. or less at a rate of over 4°C./s in the foregoing process step.

An embodiment of the invention involves (vi) tempering of steel partsformed in the foregoing process step by applying a secondary heattreatment at a temperature of 250° C. or less, and obtaining at least90% martensitic microstructure.

An embodiment of the invention is (vi) the tempering of the formed steelparts at a temperature between 140° C.-200° C. by applying a secondaryheat treatment for 2-8 hours and obtaining at least 90% martensiticmicrostructure in the foregoing process step.

In an embodiment of the invention, (vi) three-dimensional steel partsobtained after the foregoing process step have a hardness of at least480 HB, a tensile strength of at least 1700 MPa, a total elongation ofat least 7% and/or an impact strength of at least 16 J.

An embodiment of the invention includes (vi) cleaning the surface of theformed steel part before or after the processing step.

The structural and characteristic features and all advantages of theinvention will be understood more precisely by means of the detailedexplanations and figures hereinbelow; accordingly, the evaluation shouldbe in line with this detailed explanation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1F The phase transformation graphs obtained in the methodaccording to the invention during cooling process at the cooling ratesof the C-001 material produced as hot rolled plate (Ms: Martensitestarting temperature, Mf: Martensite final temperature, Bs: Bainitestarting temperature)

FIG. 2 Electron microscope image of the C-001 sample after hot formingand tempering processes

DETAILED DESCRIPTION OF THE EMBODIMENTS

Alloy composition and production method for the production of hot-formedarmor steel according to the invention is explained in this detaileddescription in order to better understand the subject with its preferredembodiments, and in a way that does not have any restrictive effect.

The invention is predicated on the development of an iron-based alloycomposition for obtaining three-dimensional parts from armor steel hardenough to endure hot forming and which has a ballistic protectionfeature, and the optimization of the hot forming method using thiscomposition.

Iron-based alloy composition according to the invention basicallyincludes;

0.28-0.34% carbon (C),

max 0.25% silicon (Si),

max 0.8% manganese (Mn),

0.85-0.95% chromium (Cr),

1.10-1.50% nickel (Ni),

0.41-0.50% molibden (Mo),

0.001-0.007% boron (B),

0.002-0.03% niobium (Nb), and

balanced amounts of iron and inevitable impurities.

According to one embodiment of the invention, the iron-based alloycomposition also may contain one or more unenviable elements selectedfrom the group containing phosphorus (P), sulfur (S), copper (Cu),aluminum (Al), tungsten (W), cobalt (Co), titanium (Ti), oxygen (O),hydrogen (H), nitrogen (N).

Within the scope of the invention, the alloy formed with the chemicalcomposition described above is transformed from liquid steel form tosolid steel form by ingot casting or continuous casting, thus, castingprocess is performed and the steel is casted into ingot or slab inthree-dimensional armor steel hot forming method. The slab or ingot isheated above 1050° C.—preferably to 1200° C.—for at least 4 hours andthen hot rolled into a plate.

Hot rolled plate is cooled down to 2° C./s or slower; therefore, itsmicrostructure includes ferrite+pearlite, bainite, or a mixture of thesephases. The plate is heated above 300° C. and its microstructure istransformed into tempered martensite, provided that the process isperformed faster without cooling process. In conclusion, if the desiredslow cooling rates are achieved, a plate with a hardness scale of 300 HBis obtained.

Hot rolled plate is cut into desired forms by means of CNC, flame, waterjet, laser, saw, etc., and the cut plates are subjected to primary heattreatment for at least 10 minutes by heating to a temperature below1000° C., and above Ac₃. The heated plate, thereafter, is placed in awater-cooled tool in a press while it is still hot.

The hot plate is shaped by cooling it to a temperature of 300° C. orbelow so that a martensitic microstructure is obtained at a speed above4° C./s by means of the force applied by the press and the water-cooledtool in the press.

Three-dimensionally formed steel part is removed from the tool andtempered by applying a secondary heat treatment at a temperature of 250°C. or below, and tempered martensitic microstructure is obtained. Thepart surface is cleaned by sandblasting, polishing, etc.

Three-dimensional steel parts produced by means of the method describedabove provide hardness of at least 480 HB, tensile strength of at least1700 MPa, total elongation of at least 7% and/or impact strength of atleast 16 J, and can be used as armored parts with ballistic resistance.

By means of the recommended method according to the invention, the platein the chemical composition developed for the steel alloy is producedwith a microstructure consisting of ferrite+perlite, bainite or amixture of these phases, after a cooling process of 2° C./s or slowerupon the hot rolling. The microstructure of plate is transformed intotempered martensite when performed faster by heating the plate up to atemperature of 300° C. and above, without the need for a coolingprocess. The plate produced in this way has a hardness scale below 300HB and does not yet have a ballistic resistance, making it easier to becut in the desired form. The microstructure of the plates cut in desiredsizes is transformed into martensite by means of being heated to atemperature below 1000° C. and austenitized and then placed in the toolfor the forming process, and obtaining a three-dimensional form in thetool with the help of a press and cooling the tool from outside withwater. The austenite begins to transform into martensite at atemperature slightly above 300° C. The part with the desired form can beremoved from the tool below this temperature. The surface of thethree-dimensional steel part can be flattened via sanding and polishing,after a cooling process at room temperature. Surface cleaning processrefers to the cleaning of the surface up to a depth of 100 microns. Thepart is re-heated and tempered by applying heat treatment at atemperature below 250° C. for at least 1 hour at the final phase of themethod. Surface cleaning can be applied after tempering as well.

In the recommended method according to the invention, the chemicalcomposition of the developed steel has been designed in such a way thatits microstructure can transform into martensite at cooling rates of 4°C./s or higher during cooling, and thus martensitic structure can beobtained at relatively low cooling rates observed in thick-sectionedparts. Produced steel includes at least 90% martensite microstructurallyafter hot forming and press hardening processes. The carbon amount inthe designed iron-based alloy composition is between 0.28%-0.34%,providing a high weldability in the produced steel. Its manganese amountis below 0.8% in order to prevent segregation. The chromium amount islimited between 0.85%-0.95% so as to delay the perlite formation duringcooling process, and to ensure high hardenability. The nickel amount isoptimized between 1.10%-1.50% and the molybdenum content between 0.41%and 0.50% in order to increase the hardening depth and hardenabilityfeatures to provide ballistic properties. Silicon content is limitedbelow 0.25% in order to prevent the formation of silicon oxide in hotrolling and heat treatment processes.

Welding the armor steels including high amount of carbon with otherparts in the vehicle production causes defects in the weld zone. Forthis reason, this process is not desired by armored vehiclemanufacturers. Furthermore, cracks may occur due to thermal stress thatoccurs during tool cooling in high carbon steels and stresses caused byBain strains occurring with martensite transformation. The presentinvention provides the necessary armor feature with a carbon amount of0.28%-0.34% to eliminate these problems.

It is known that steels with high manganese content convert into aninhomogeneous structure due to manganese segregation after hot rolling.Even though manganese intensifies the hardenability of the steel, crackformation in continuous casting during steel production causes hardshipssuch as routing, etc. For this reason, manganese amount was limitedbelow 0.8% in the present invention. Furthermore, high silicon contentcauses silicon-based oxide formation on the surface of the steel afterhot rolling or during heat treatment. This oxidation cannot be removedby acid and sanding. It, therefore, constitutes a problem for latercoating or painting. Fayalite (Fe₂SO₄) is formed during hot rolling ofsteels containing silicon, and bonds with FeO. Since this is a strongbond, it makes it difficult to remove the oxide and causes the formationof red scale. The scale cannot be removed by traditional oxide removalmethods, causing defects in these areas and decreasing the paint coatingworkability. It is known in the present art that there is no Fe₂SO₄compound formation, which causes red scale formation when below 0.25%.Hard oxide layers formed on the surface during hot forming damage andreduce the lifetime of the tool, besides the paint coating workability.Therefore, the silicon amount of the alloy developed in compliance withhot forming was kept below 0.25%. Hereby, its surface properties arealso improved. However, silicon affects solid solution hardeningpositively and raises hardenability. Silicon can be used to preventcarbide formation in steels. Decreasing of hardenability dramaticallyaffects the ballistic properties by enabling formation of unwantedphases during cooling. Hence, 0.41% molybdenum alloying is also employedto increase hardening. Hence, the alloy according to the invention isunique on this sense.

Iron-based alloy composition developed according to the invention has astructure that can provide ballistic properties with hot forming andtool cooling, and subsequent tempering. Due to low amount of silicon, nored scale formation is observed on the surface of the steel, and theoxidation layer can be, thereby, easily removed. Therefore, it is amaterial with high paint coating workability. The hardness scale of thefinal product is at least 480 HB, its tensile strength is typically atleast 1700 MPa, its total elongation value is at least 7%, and thenotched impact toughness value is at least 16 J at the room temperature.

Tests and analyzes were carried out with hot formed armor steel partsamples obtained within the scope of the invention, and comparativeresults were recorded and presented in tables below.

Compositions of the hot formed armor steel samples developed within thescope of the invention are presented in Table 1. Phase transformation atdifferent cooling rates for C-001 alloy is demonstrated in FIGS. 1A-1F.It is evident that bainite is formed prior to martensite transformationwhen the alloy is cooled down to 2° C./s or slower. Therefore, beforehot forming process, the material should be cooled down to 2° C./s orslower after hot rolling in order to be easily cut to the desireddimensions. The hardness scale of the material produced by cooling inthis manner is below 300 HB. Mechanical properties of armor steelsproduced by heating at 900° C. for 10 minutes and via cooling in thetool and subsequent tempering are presented in Table 2. It is evidentthat the hardness scale above 500 HV are obtained after hot forming andtempering processes. Ballistic performance values of different alloysproduced by hot forming and tempering are given in Table 3 and Table 4after being tested with different ammunitions. The scanning electronmicroscope image of the C-001 sample after hot forming and temperingprocesses is given in FIG. 2. It is evident that a martensitic structurehas been obtained. It was observed that the ballistic performance of theH009 alloy—whose composition is presented in Table 1—is not sufficient,even though it has the highest impact toughness value. C and Mo contentsof the H009 alloy are lower than the other alloys, while its Mn contentis slightly higher. The H010 alloy, which is very similar to the H009alloy and has only a slightly higher C amount, has shown a highballistic performance against the 7.62×51 Nato Ball ammunition, thoughhaving a lower thickness when compared to H009 alloy. Therefore, H009alloy is excluded from the patent scope. H010 alloy with a slightlyhigher carbon amount did not display the desired performance on theballistic tests performed with the 5.56×45 mm SS109 ammunition. Hence,the C-001 alloy is developed by increasing the Cr, Mo, B and Nb amountsof this alloy and decreasing the Mn amount albeit. This alloy isproduced via vacuum melting method, differing from other alloys beingproduced by melting under Ar protection under atmospheric conditions.Therefore, it is ensured that amounts of N, O, H elements and therelevant inclusions are reduced in steel, and the casting cavities arelargely eliminated. The ballistic performance of the C-001 alloyproduced in different thicknesses are demonstrated in the Table 4 afterballistic tests performed with 7.62×51 Nato Ball ammunition. It isevident that this alloy provides ballistic strength even at lowerthicknesses compared to other alloys. The ballistic performance of theC-001 alloy is demonstrated in the Table 5 after ballistic testsperformed with 5.56×45 SS109 ammunition, and the desired protectionlevel is reached. For this reason, C-001 alloy is a patented chemicalcomposition. Nonetheless, H-009.5 alloy is also patented as it providesthe desired protection level in 7 mm thickness. Due to similarproduction methods, it is regarded that the N, O, H amounts of theH009.5 alloy are similar to the H010 alloy. C-001 alloy is produced byvacuum melting method. Therefore, the N, O, H amounts are lower. Therestricted amounts of N, O, H elements are, thus, considered to beineffective regarding the ballistic performance. However, due tophenomena such as hydrogen embrittlement and grain boundary corrosionthat may arise problems over time, it is aimed to keep the amount ofthese elements low even if no effect on ballistic performance isobserved. No significant effect of Al, S, P elements have been observedagainst ballistic performance. In the steels produced at the presenttime, boron addition is limited to 10-20 ppm (Sharma, M., Ortlepp, I., &Bleck, W. (2019). Boron in Heat-Treatable Steels: A Review steelresearch international, 90(11), 1900133). Nevertheless, it is possibleto achieve a higher amount of boron addition with necessary precautions.There are different reasons for this limitation. Considering themanufacturability, in the boron addition phase of the steel, nitrogenforming elements should be bound with nitrogen and B₂O₃ formation shouldbe prevented by keeping the amount of BN and oxygen low. This problemwas tried to be avoided with the addition of high amount of Nb (246 ppm)and Al (160 ppm) in C-001 steel and limiting the N (59 ppm) and O (45ppm) amounts. Otherwise, it is inevitable for free B atoms to form BN.Boron carbides can be formed in boron added steels to some extend evenwhen deemed protection is applied, but their stability is low and theydissolve at temperatures above 800° C. Excessive boron addition (>80ppm) causes hot shortness. It is possible to work with lower amounts ofboron in terms of manufacturability. Furthermore, boron element can bediverged from steel in the phenomenon called “boron fade” when beingheated above 900° C. The hardenability is also affected in such a case.This patent relates to thick-sectioned parts; thus, this risk may occurat near-surface, just as decarbonization. According to thermodynamiccalculations, up to 41.9 ppm B can dissolve in austenite. This amountmay rise up to 97.4 ppm in delta ferrite during solidification.Interstitially or substitutionally dissolving boron generally segregatesto grain boundaries and near regions. It increases the hardenability bydelaying ferrite or perlite nucleation in these regions. Regarding thehardenability, the boron being in dissolved form or in fine precipitatesis considered suitable for a high hardenability. Hardenability decreaseswith excessive boron and coarse boron-based carbide formation.Nevertheless; in the present case, boron carbides are regarded asdissolved due to quenching process performed in approximately 1000° C.There are publications in terms of toughness regarding that the boronelement at the grain boundary decreases the toughness, while there arealso papers claiming that it has no effect at all Toughness is generallydependent on steel alloy and is highly related to the toughness levelexpected from steel. For example, if there is no Al, Ti, Nb and similarelements that can form nitride, BN precipitates can cause austenitegrain coarsening and reduce toughness. Therefore, the minimum-maximumamounts of these elements have been determined based on the alloysdemonstrated in the Table 1. W, Co, Cu, Ti, Al, S, P elements wereobserved in trace amounts in 4 different alloys, and no effect onballistic resistance have been observed. The relevant values per alloyare also given in Table 1.

TABLE 1 Hot formed armor steel compositions measured by OES before hotstamping process Amount of Element Sample (by weight - %) Code C Si Mn PS Cr Mo Ni W Co Cu H009 0.26 0.11 0.88 0.080 0.020 0.79 0.39 1.27 0.0250.01 0.045 H009.5 0.30 0.22 0.73 0.080 0.020 0.88 0.41 1.38 0.034 <0.0050.083 H010 0.34 0.19 0.78 0.070 0.020 0.77 0.37 1.25 0.018 <0.005 0.08C-001 0.33 0.18 0.70 0.007 0.001 0.91 0.43 1.25 0.029 <0.005 0.064Amount of Element Sample (by weight - %) Code Ti Al B Nb N O H Fe H0090.003 0.0045 0.0013 0.0141 n/a n/a n/a Rest H009.5 <0.002 0.0060 0.00230.0095 n/a n/a n/a Rest H010 0.003 0.0038 0.0012 0.0156 0.0114 0.01440.000058 Rest C-001 <0.002 0.0160 0.0064 0.0246 0.0059 0.0045 0.000088Rest

TABLE 2 Mechanical properties of developed armor steels after hotforming and tempering. Impact Tensile Total Micro Toughness StrengthElongation Hardness [J] [MPa] [%] [HV] Sample Std Std Std Std Code* MeanDeviation Mean Deviation Mean Deviation Mean Deviation H009 64 14 16064.5 10.9 1.2 512 14 H010 41.1 0.70 2074 23.1 11.30 1.2 584 21 C-001 41.80.2 1838.6 37.1 11.1 3.0 553 11

TABLE 3 Ballistic test results of the H009, H009.5 and H010 materialsproduced by hot forming and tempering. Measured Measured 7.62 mm × 515.56 mm × 45 Sample Sample Nato Ball Nato SS109 Sample Size ThicknessSpeed Speed Penetration Firing Code [mm] [mm] [m/s] [m/s] after Test 1H010 100 × 200 6.0 845.74 — NONE 2 — 912.82 NONE 3 — 959.76 OBSERVED 1H009.5 100 × 200 7.0 842.71 — NONE 2 — 904.53 NONE 3 — 956.99 NONE 1H009 100 × 195 6.4 839.72 — OBSERVED 2 857.02 — OBSERVED 3 847.08 —OBSERVED

TABLE 4 Ballistic test results of C-001 material produced by hot formingand tempering in different thickness. Sample Thickness Rate of 7.62 mm ×51 Nato Ball code [mm] fire Speed [m/s] Test result C-001 5.7 1 840 Nopenetration 2 843 No penetration C-001 6.0 1 830 No penetration

TABLE 5 Ballistic test results of the C-001 material produced by hotforming and tempering in 6.5 mm thickness. Measured Measured 7.62 mm ×51 5.56 mm × 45 Sample Sample Nato Ball Nato SS109 Sample Size ThicknessSpeed Speed Penetration Firing Code [mm] [mm] [m/s] [m/s] after Test 1C-001 100 × 200 6.5 965 NONE 2 — 956 NONE 3 — 955 NONE 4 962 NONE 5 963NONE 6 971 OBSERVED 7 842 NONE

What is claimed is:
 1. An iron-based alloy composition developed forproducing a hot formed armor steel, comprising 0.28-0.34% carbon, max0.25% silicon, max 0.8% manganese, 0.85-0.95% chromium, 1.10-1.50%nickel, 0.41-0.50% molybdenum, 0.001-0.007% boron, 0.002-0.03% niobiumby weight and a balanced amount of iron and inevitable impurities. 2.The iron-based alloy composition according to claim 1, comprising one ormore elements selected from the group containing trace amounts ofphosphorus, sulfur, copper, aluminum, tungsten, cobalt, titanium,oxygen, hydrogen, and nitrogen.
 3. A hot formed armor steel, comprising0.28-0.34% carbon, max 0.25% silicon, max 0.8% manganese, 0.85-0.95%chromium, 1.10-1.50% nickel, 0.41-0.50% molybdenum, 0.001-0.007% boron,0.002-0.03% niobium by weight and a balanced amount of iron andinevitable impurities in a composition of the hot formed steel.
 4. Thehot formed armor steel according to claim 3, comprising one or moreelements selected from the group containing trace amounts of phosphorus,sulfur, copper, aluminum, tungsten, cobalt, titanium, oxygen, hydrogen,and nitrogen.
 5. The hot formed armor steel according to claim 3,wherein the hot formed armor steel has a hardness of at least 480 HB, atensile strength of at least 1700 MPa, a total elongation of at least7%, and/or an impact strength of at least 16 J.
 6. The hot formed armorsteel according to claim 3, comprising at least 90% martensite in amicrostructure of the hot formed armor steel.
 7. A hot formed armorsteel production method, comprising the following steps: i. an ingot orslab casting of an alloy comprising 0.28-0.34% carbon, max 0.25%silicon, max 0.8% manganese, 0.85-0.95% chromium, 1.10-1.50% nickel,0.41-0.50% molybdenum, 0.001-0.007% boron, 0.002-0.03% niobium byweight, and balanced amounts of iron and inevitable impurities to obtainan ingot or a slab, ii. hot rolling the slab or the ingot into a plate,iii. plate cooling and cutting, iv. applying a primary heat treatment tocut the plate, v. forming the plate by pressing in a cooled tool, vi.applying a secondary heat treatment to formed steel parts.
 8. The hotformed armor steel production method according to claim 7, comprisingone or more elements selected from the group containing trace amounts ofphosphorus, sulfur, copper, aluminum, tungsten, cobalt, titanium,oxygen, hydrogen, and nitrogen in the step i.
 9. The hot formed armorsteel production method according to claim 7, comprising heating theslab or the ingot to above 1050° C. for at least 4 hours.
 10. The hotformed armor steel production method according to claim 7, comprisingcooling the plate down to 2° C./s or slower, a microstructure offerrite+perlite, bainite, or a mixture of phases and obtaining a platewith a hardness scale below 300 HB, a heating process of the plate above300° C. and transforming a microstructure of the plate into a temperedmartensite, provided that the heating process is performed fasterwithout cooling in the step iii.
 11. The hot formed armor steelproduction method according to claim 7, wherein the primary heattreatment for cutting the plate is performed by heating the plate to atemperature below 1000° C. and above AC3 for at least 10 minutes in thestep iv.
 12. The hot formed armor steel production method according toclaim 7, comprising forming the plate by cooling the plate to atemperature of 300° C. or less at a rate of over 4° C./s in the step v.13. The hot formed armor steel production method according to claim 7,comprising tempering of the formed steel parts in the step vi byapplying the secondary heat treatment at a temperature of 250° C. orless, and obtaining at least 90% martensitic microstructure.
 14. The hotformed armor steel production method according to claim 7, comprisingtempering of the formed steel parts at a temperature between 140°C.-200° C. by applying the secondary heat treatment for 2-8 hours andobtaining at least 90% martensitic microstructure in the step vi. 15.The hot formed armor steel production method according to claim 7,wherein three-dimensional steel parts obtained after the step vi has ahardness of at least 480 HB, a tensile strength of at least 1700 MPa, atotal elongation of at least 7%, and/or an impact strength of at least16 J.
 16. The hot formed armor steel production method according toclaim 7, comprising cleaning surfaces of the formed steel parts beforeor after the step vi.